The present invention concerns a method of transmitting data. It is also directed to a base transceiver station using this method.
In the field of digital mobile radio, the speech or other data transmitted by a mobile station is received by a fixed base transceiver station (or base station). The data signalling rate of the data transmitted by radio, i.e. "on air", is lower than that of data transmitted "by wire" in the various fixed units of the radio system (for example, speech is transmitted on air at the full data signalling rate of 13 kbit/s and data is transmitted at 12, 6 or 3.6 kbit/s while the data signalling rate by wire is 64 kbit/s). Consequently, a transcoder (referred to in the GSM standard as a TRAU: Transcoder/Rate Adaptor Unit) is used to adapt the data signalling rate of the data transmitted on air to that of the data transmitted by wire.
Data is usually transmitted by a mobile station to a base station in the form of streams of frames. After a traffic channel designed to convey speech or data is activated for the exchange of data between the base station, and a mobile station the transmission of the first streams of data frames from the mobile station to the base station may give rise to an offset relative to that corresponding to steady-state conditions.
This phenomenon is illustrated in the example shown in FIG. 1, which is a timing diagram indicating the organization under the GSM standard of the streams of TA data frames (air frames) as prepared by the mobile station for transmission to the base station and that of corresponding processed data frames TRAU transmitted by the base station to the transcoder.
The (speech or other) data frames to be transmitted at the full data signalling rate by the mobile stations in systems to the GSM standard are initially arranged in groups G1, G2, G3, each comprising a stream of five frames T1 followed by two streams of four frames T2 and T3, these groups of 13 frames being repeated periodically to constitute multiframes M1, M2 each comprising 26 frames (this is in systems to the GSM standard). These groups are shown in FIG. 1 in which rectangles symbolically represent the streams of frames T1, T2, T3, the number of frames in each stream being shown in the respective rectangle.
The mobile station transmits these streams of frames in the form of a continuous sequence of frames (not shown) after interleaving, i.e. after combination of R1 through R7 type sequences each comprising two consecutive frame streams and consequently eight or nine frames in the case of speech data (the interleaving may be different for data other than speech data), two consecutive sequences of the set R1 through R7 overlapping over a frame stream length.
At the start of transmission, i.e. after activation of a transmission channel between the mobile station and the base station, the mobile station can begin to transmit only at the start of a T1, T2 or T3 type stream. To this end its internal clock is synchronized to that of the base station.
If the mobile station were to begin to transmit at any other time, the base station would not be able to reconstitute from the data received the initial streams of frames as present at the mobile station before interleaving and would therefore be incapable of using the data transmitted.
From the streams of frames received from the mobile station, the base station reconstitutes the initial structure with the initial frame groups G1, G2, G3 and in turn transmits this data to the transcoder in 20 ms packets constituting so-called TRAU frames, starting at the end of reception of a first sequence of eight or nine frames after activation of the channel (20 ms is the duration of the data packets used in wire networks).
In the GSM standard, a time period of 20 ms is equivalent to the duration of four and one-third frames. Thus one of the following three situations is encountered at the start of transmission:
if the first TA frame received and identified (i.e. recognized after reconstitution of the original structure) by the base station after activation of an on air receive channel belongs to an R1, R4 or R7 type sequence, or more generally an R(3n+1) type sequence, i.e. if it is the first frame of a T1 stream, a stream of S1 type TRAU frames is transmitted by the base station to the transcoder and there is no "gap" in transmission; PA1 if the first TA frame received and identified after activation belongs to an R2 or R5 type sequence, or more generally an R(3n+2) type sequence, i.e. if it is the first frame of a T2 stream, a train of S2 type TRAU frames is transmitted by the base station to the transcoder and a one-third frame "gap" T.sub.1/3 appears between the third and fourth frames during which the base station does not transmit any usable data to the transcoder; PA1 if the first TA frame received and identified after activation belongs to an R3 or R6 type sequence, or more generally an R(3n+3) type sequence, i.e. if it is the first frame of a T3 stream, a stream of S3 type TRAU frames is transmitted by the base station and a two-thirds frame "gap" T.sub.2/3 appears between the third and fourth frames. PA1 after activation of a channel of the base station for reception of said data frames, the first stream of frames received on said channel by said base station is associated with a reference number within the corresponding group of frames, PA1 transmission of data by said base station to said transcoder is then delayed by an amount depending on the reference number in said group of frames of said first stream of frames, whereby a continuous stream of data is transmitted by said base station to said transcoder.
This phenomenon is due to the fact that the data frames are not necessarily transmitted on air by the mobile stations immediately after the activation of a receive channel and in systems to the GSM standard this results in practice in an absence of data transmitted to the transcoder for 1.5 or 3 ms.
It is evident that this phenomenon occurs only at the start of transmission and that after some particular time (60 ms in the FIG. 1 example) the transmission "gaps" are eliminated.
If the data transmitted is speech data the presence of these "gaps" in the initial transmission increases the time needed to synchronize the transcoder with the base station. During this time either the speech is unintelligible or extrapolation is required. This is naturally highly undesirable. Exactly the same problem arises in full signalling rate or half-rate transmission of speech data.
If the data transmitted is non-speech data, the GSM standard prohibits transmission of data to the transcoder during "gaps" in reception from the mobile stations; one known solution to this problem is to insert stuffing frames (comprising series of 1 s, for example) pending the next frame, which results in an additional time-delay of 17 to 20 ms. Although this solution is of little advantage in full data signalling rate transmission, because of the penalty in terms of transmission time-delays, it is totally unthinkable in half-rate transmission, especially in the case of data other than speech data, in respect of which the base station transmits to the transcoder packets having a duration of 40 ms rather than 20 ms. In the case of an R(3+n3) type sequence, a "gap" would then occur in the middle of a packet of data; stuffing is not feasible in this case because the stuffing bits occurring in the middle of the packets would be taken as data bits.
The same problem could arise in the case of half-rate speech transmission in which case speech packets of greater than 20 ms duration would be sent by the base station.